1. Field of the Invention
The present invention relates to an image forming apparatus, which transfers to a transfer member such as a recording sheet or an intermediate transfer belt, visual images that are formed on a surface of image carriers such as rotatably driven photosensitive drums.
2. Description of the Related Art
In an image forming apparatus, a displacement of dots occurs due to decentering of a driven gear, which transmits a rotary driving force to image carriers. To be specific, a decentering of the driven gear, which rotates on the same axial line as the image carriers, results in occurrence of a speed fluctuation of an attribute that causes the image carriers to draw a sine wave equivalent to one cycle per single rotation of the image carriers. The sine wave equivalent to one cycle occurs due to the following reasons. When a driving side gear is meshing with the longest diameter portion of the decentered driven gear, a linear speed of the image carriers becomes the slowest and when the driving side gear is meshing with the shortest diameter portion of the decentered driven gear, the linear speed of the image carriers becomes the fastest. Because the longest diameter portion and the shortest diameter portion are positioned at locations that are 180° apart from each other, the speed fluctuation per single rotation of the image carriers resembles the sine wave equivalent to one cycle. When the linear speed has increased than the original linear speed, dots formed on the image carriers reach a transfer position at an earlier timing than the original timing. Similarly, when the linear speed has decreased than the original linear speed, the dots formed on the image carriers reach the transfer position at a later timing than the original timing. Such a difference in the timings results in the displacement of the dots.
In the image forming apparatus, after forming visual images of different colors on the respective image carriers, the visual images are superposed on a transfer member and transferred to obtain a multicolor image. If the displacement of the dots mentioned earlier occurs in such an image forming apparatus, a color drift is noticeable in the multicolor image due to the displacement of the dots of the respective colors.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-47920 uses a technology to reduce the color drift. In other words, the image forming apparatus forms a speed-fluctuation detecting image that includes a plurality of toner images that are arranged at a predetermined pitch in a surface moving direction, at a predetermined timing, on a surface of the respective image carriers in the form of photosensitive drums. After transferring the speed-fluctuation detecting image on a transfer belt, photo sensors are used to detect each toner image in the transfer belt shaped speed-fluctuation detecting image, and based on detection intervals, a speed fluctuation pattern per single rotation of the respective photosensitive drums is detected. Next, based on the respective speed fluctuation pattern, a driving speed fine tuning pattern (for example, a pattern that is antiphase with respect to the speed fluctuation pattern and that includes an equal amplitude and cycles) is specified that enables to negate the speed fluctuation of the photosensitive drums. The driving speed fine tuning pattern is specified for each of the respective photosensitive drums. Next, when forming the image based on image data that is transmitted from a personal computer etc., each photosensitive drum is driven using the respective prior specified driving speed fine tuning pattern, thus causing each photosensitive drum to surface move with a stable speed. Due to this, the color drift due to the displacement of the dots of each color visual image can be reduced.
When a driving motor, which drives the respective image carrier, is constantly driven in the same speed mode (average speed per single rotation), the speed fluctuating pattern per single rotation of the image carrier due to the decentering of the driven gear etc. remains constant unless the image carrier is removed with the driven gear. However, if a linear speed setting of the image carrier is modified along when switching from a low speed printing mode to a high speed printing mode, because a rotation cycle of the image carrier is changed, the speed fluctuation pattern also changes. Detecting the speed fluctuation pattern for each mode increases a waiting time period of a user.
Further, in recent high image quality enabled devices, for curbing a marginal displacement of the dots due to minute expansion of components along with environmental variations or due to displacement of the components by an external force, a marginal linear speed difference is maintained between the respective image carriers. A misalignment detecting image that includes dots of various colors is periodically formed, sensors are used to detect a marginal displacement of the dots of each color in the misalignment detecting image, and the linear speed difference is corrected based on a detection result of the marginal displacement. However, the speed fluctuation pattern of each image carrier changes at the time of correcting the linear speed difference. During every correction of the linear speed difference, forming the speed-fluctuation detecting image and detecting the speed fluctuation pattern using the sensors significantly increases the waiting time period of the user.